If you wish to create a Multiplayer RPG then you need to figure out alot of things almost before you even hit the drawing board.
System Specifications play a very large role in what you can hope to accomplish.
As RPG players on general don't have powerful PCs unlike FPS players you have to cater to this, so the first step is setting a base systems specification.
What I would recommend would be:
500MHz Processor
128MB Ram
TnL Compatible 32MB Direct3D Card (Geforce2 for example)
500MB Hard Disk (trust me size is a HUGE issue in RPGs)
640x480x16bpp Base Screen Size
Broadband 128Kb Connection (ISDN/xDSL)
Keyboard & 2-Button Mouse
Of course you can go higher or lower, but as most people are on Windows XP this should be roughly what they're using.
A TnL Card allows you to push almost double the polygons you can on a standard card, so without it you'd be looking at a 1GHz processor minimum.
As for the connection speed, this is also important. Not so much for data being transfered but for
ping rates. The lower the ping the faster the connections reaction which is important for online games.
56K = 250-500ms
ISDN= 150ms
DSL = 40-80ms
T1 = <10ms
That is roughly how it works, of course some people will have better lines than others so this can vary. AOL 56K v92 around London for example you could get a ping of 180-250ms which I liked, but around Los Angeles it'll be somewhere around 400-800ms which is unplayable.
Best thing for this is actually setup a utility to measure ping, so that people know roughly what they're connection is capable of and then cater to that. I would aim for 150ms for game speed.
Why does ping matter?
Because this can really give people with better connections an unfair advantage. It can also mean that if the game is working too fast for the user you can see incoherant bugs appear.
Such-as getting stuck in walls and alike as the Server isn't capable of understanding where you are properly.
So what you have to do is build a system upon this which caters for all of these connection speeds.
Now in order to do this you have to build the game in a strange way.
It is called a Server-Client model.
What this means is that every aspect of your game is coded in sections, some of which are used by the user, some are used by the server and some are used by both.
Well this is unless you choose to create 2 seperate programs, one for a server and one for the client.
Still you end up with many functions used by both and thus you work from a base which allows you to basically plugin new sections of code.
The way to do this is through careful planning.
Client -> Movement -> Collision
...................-> Interaction
.......-> Battling -> Collision
...................-> Status Upgrades
.......-> Inventory-> Status (read only)
...................-> Items
...................-> Map
Server -> Movement -> Current Position
...................-> Interaction
.......-> Battling -> Interaction
...................-> Stats Update
.......-> Inventory-> Status (read/write)
...................-> Items
...................-> Map
these also coincide with each other so they're all interacting but that is a basic base which you can work from.
Each of those are sections of code, which then have sections of code.
It's like a tree you move down and code only a small bit just for an effect which is part of a bigger section of code as a whole.
Coding like this allows you to make the entire program timer based.
So movement and everything can be simply placed into a timer section which is run every Nth loop, rather than requiring you to use a timer solution for everything.
The main achievement to get a basic RPG base working.
You can start working on the concept for the game and what each section will actually do, suchas how the interation will work, how the battling will, etc...
Remember no code has actually been created yet.
Most games are 90% planning, 8% development, 2% cursing at your monitor
I will be back later with some further help.
Just incase your wondering, I'm currently working on 2 Multiplayer Games. One of which is an Online RPG (not an MMORPG cause it isn't designed for thousands of players at once, personally I think it takes away from the community style of games).
So I am in the same boat right now... difference being is I understand how to work on all of this

AthlonXP 2500+ | 256MB DDR PC2700 | GeForce FX 5700 60.18 | DirectX 9.0b | CMedia 8620 | Windows XP Professional